1. Field of the Invention
The present invention relates to apparatus and methods for simulating the acoustical effects of a localized sound source.
2. Description of the Prior Art
Directional audio systems for simulating sound source localization are well known to those skilled in audio engineering. Similarly, the principal mechanisms for sound source localization by human listeners have been studied systematically since the early 1930's. The essential aspects of source localization consist of the following features or cues:
1) Interaural time difference--the difference in arrival times of a sound at the two ears of the listener, primarily due to the path length difference between the sound source and each of the ears. PA0 2) Interaural intensity difference--the difference in sound intensity level at the two ears of the listener, primarily due to the shadowing effect of the listener's head. PA0 3) Head diffraction--the wave behavior of sound propagating toward the listener involves diffraction effects in which the wavefront bends around the listener's head, causing various frequency dependent interference effects. PA0 4) Effects of pinnae--the external ear flap (pinna) of each ear produces high frequency diffraction and interference effects that depend upon both the azimuth and elevation of the sound source. PA0 1) The existing schemes either use extremely simple models which are efficient to implement but provide imprecise localization impressions, or extremely complicated models which are impractical to implement. PA0 2) The artificial localization algorithms are often suitable only for headphone listening. PA0 3) Many existing schemes rely on ad hoc parameters which cannot be derived from the physical orientation of the source and the listener. PA0 4) Simulation of moving sound sources requires either extensive parameter interpolation or extensive memory for stored sets of coefficients.
The combined effects of the above four cues can be represented as a Head Related Transfer Function (HRTF) for each ear at each combination of azimuth and elevation angles. Other cues due to normal listening surroundings include discrete reflections from nearby surfaces, reverberation, Doppler and other time variant effects due to relative motion between source and listener, and listener experience with common sounds.
A large number of studio techniques have been developed in order to provide listeners with the impression of spatially distributed sound sources. Refer, for example, to "Handbook of Recording Engineering" by J. Eargle, New York: Van Nostrand Reinhold Company, Inc., 1986 and "The Simulation of Moving Sound Sources" by J. Chowning, J. Audio Eng. Soc., vol. 19, no. 1, pp. 2-6, 1971.
Additional work has been performed in the area of binaural recording. Binaural methods involve recording a pair of signals that represent as closely as possible the acoustical signals that would be present at the ears of a real listener. This goal is often accomplished in practice by placing microphones at the ear positions of a mannequin head. Thus, naturally occurring time delays, diffraction effects, etc., are generated acoustically during the recording process. During playback, the recorded signals are delivered individually to the listener's ears, by headphones, for example, thus retaining directional information in the recording environment.
A refinement of the binaural recording method is to simulate the head related effects by convolving the desired source signal with a pair of measured or estimated head related transfer functions. See, for example U.S. Pat. No. 4,188,504 by Kasuga et al. and U.S. Pat. No. 4,817,149 by Myers.
The two channel spatial sound localization simulation systems heretofore known exhibit one or more of the following drawbacks:
A need remains in the art for a straightforward localization model which uses control parameters representing the geometrical relationship between the source and the listener to create arbitrary sound source locations and trajectories in a convenient manner.